Radiotelephone system



oo 5 3w. moEmmEo I. MOLNAR RADIOTELEPHONE SYSTEM Attorneys moi 059 is V235 Sept. 26, 1950 Filed May 9, 1947 Patented Sept. 26, 1950 RADIOTELEPHONE SYSTEM Imre Molnar, Chicago, Ill., assignor to Automatic Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application May 9, 1947, Serial No. 746,929

8 Claims.

The presentinvention relates to radiotelephone systems and more particularly to such systems involving exchanges of the automatic dial type.

In prior radiotelephone systems, connections have been extended from calling subscriber substations by way of automatic switching apparatus to a first manual operator switchboard and then extended by the operator thereat over radio links to a second manual operator switchboard and then extended by the operator thereat by way of automatic switching apparatus to called subscriber substations. In these systems the radio links are employed entirely on a manual basis by the operators at the two terminating manual switchboards. While systems of this character are entirely satisfactory in operation, large switchboards, staffed by a great number of operators, arerequired in order to complete the calls between the subscriber substations in the various automatic exchanges of the system.

Accordingly, it is an object of the present invention to provide a radiotelephone system involving an automatic exchange; wherein connections are extended over the radio links to the automatic switching apparatus in the automatic exchange and then the automatic switching apparatus may be operated to complete the con nections to the called subscriber substations therein, and wherein calls initiated at the subscriber substations in the automatic exchange 0 are first extended to the automatic switching apparatus therein and then the automatic switching apparatus is operated to extend the calls over the radio links.

Another object of the invention is to provide in a radiotelephone system, improved automatic switching apparatus that may be selectively operated over an associated radio link to extend a connection.

Another object or the invention is to provide in a radiotelephone system, an improved radio link capable of controlling the operation of associated automatic switching apparatus and responsive to the usual audible signals returned from the automatic switching apparatus incident to the various switching operations thereof.

Another object of the invention is to provide in a radiotelephone system, improved switching apparatus that is capable of seizing and controlling the operation of an associated radio link.

Another object or" the invention is'to provide in a radiotelephone system, an improved radio link controllable from associated automatic switching apparatus and responsive to the usual audible signals forwarded from, the automatic 2 switching apparatus incident to the various switching operations thereof.

A further object of the invention is to provide in a radiotelephone system comprising a manual exchange and an automatic exchange interconnected by radio links, an improved arrangement whereby connections may be extended from an operator switchboard in the manual exchange over the radio links to called subscriber substations in the automatic exchange, and connections maybe extended from calling subscriber substations in the automatic exchange over the radio links to the operator switchboard in'the manual exchange.

A further object of the invention is to provide in a radiotelephone system comprising a plurality of automatic exchanges interconnected by radio links, an improved arrangement whereby calling subscriber substations in a, first automatic exchange may extend connections via the associated automatic switching apparatus over the radio links to the automatic switching apparatus in a second of the automatic exchanges and then operate the last-mentioned automatic switching apparatus to extend connections to called subscriber substations in the second automatic exchange.

A still further object of the invention is to provide in a radiotelephone system, radio links comprising improved duplex control and supervisory facilities.

Further features of the invention pertain to the particular arrangement of the circuit elements of the system, whereby the above-outlined A and additional operating features thereof are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reierence to the following specification taken in connection with the accompanying drawings in which Figure 6 is a diagrammatic illustration of a radiotelephone system embodying the present invention and comprising a manual exchange Ill, anautomatic exchange 20, and two radio stations 225W and 325E; Fig. 7 is a diagrammatic illustration of a modified radiotelephone system embodying the present invention and comprising a manual exchange 30, a manual exchange 49, and two radio stations 225W and 325E; Fig. 8 is a diagrammatic illustration of a further modified radiotelephone system embodying the present invention and comprising an automatic exchange 59, an automatic exchange 69, and two radio stations 225W and 325E; and Figs. 1 to 5, inclusive, taken together illustrate the details of the apparatus incorporated in the radiotelephone system illustrated in Fig. 6 and embodying the features of the invention as briefly outlined above. In order to form a unified diagram, Figs. 1 to 5, inclusive, should be arranged from left to right in end-to-end relation.

More particularly, Fig. 1 illustrates the details of the cord circuit disposed at the operator switchboard and the details of the trunk circuit, both disposed in the manual exchange I in the radiotelephone system of Fig. 6; Fig. 2 illustrates the details of the coupling circuit disposed in the manual exchange I0 and the details of the couplin circuit disposed in the radio station 225W, and diagrammatically the F. M. radio receiver disposed in the radio station 225W; Fig. 3 illustrates diagrammatically the F. M. radio transmitter and the F. M. radio receiver disposed in the radio station 325E and the details of the coupling circuit disposed in the radio station 325E and the details of the coupling circuit disposed. in the automatic exchange 20; Fig. 4 illustrates the details of the trunk circuit disposed in the automatic exchange 20; and Fig. 5 illustrates the general arrangement of the automatic switching apparatus disposed in the automatic exchange 20.

Referring now more particularly to Fig. 6, the radiotelephone system there illustrated comprises a manual exchange I0 serving a group of subscriber substations, not shown, an automatic exchange serving a plurality of groups of subscriber substations, one of the groups including the subscriber substation T, and two spacedapart radio stations 225W and 325E respectively associated with the manual exchange I0 and with the automatic exchange 20. The manual exchange I0 also comprises an operator switchboard 500 having access to a trunk circuit IO0TC that, in turn, is connected to an associated coupling circuit 200. Also the automatic exchange 20 comprises automatic switching apparatus 500, a trunk circuit 400TC having access to and being accessible to the automatic switching apparatus 500, that, in turn, is connected to an associated coupling circuit 300. The radio station 22 5W comprises an F. M. radio transmitter 2 l 0 and an F. M. radio receiver 240, both connected to an associated coupling circuit 230. The coupling circuit 2520 in the manual exchange I0 is connected to the coupling circuit 230 in the radio station 225W by a trunk 205 incoming to the coupling circuit 230 and associated with the F. M. radio transmitter 2 I 0 and by a trunk 263 outgoing from the coupling circuit 230 and associated with the F. M. receiver 240. Similarly, the radio station 325E comprises an F. M. radio transmitter 3I0 and an F. M. radio receiver 340, both connected to an associated coupling circuit 330. The coupling circuit 300 in the automatic exchange 20 is connected to the coupling circuit 330 in the radio station 325E by a trunk 305 incoming t0 the coupling circuit 330 and associated with the F. M. radio transmitter 3I0 and by a trunk 353 outgoing from the coupling circuit 330 and associated with the F. M. receiver 340.

As shown in Fig. 1, the operator switchboard I00 comprises a number of cord circuits, including the cord circuit IOI, and standard common equipment, including a headset H3, a dial DI00 and an interrupter generator [01. cord circuit IOI comprises two plugs PI02 and Further, the

PI03, a dial key KI04 operative to connect the dial DI08 thereto, a talk key KI05 operative to connect the headset II3 thereto, and a ring key KIGB operative to connect the interrupter generator I 0? thereto. Also, the cord circuit IOI comprises a supervisory lamp LIII associated with the plug PI02, a supervisory lamp LII2 associated with the plug PI03, a supervisory relay RI associated with the supervisory lamp LI I I, a supervisory relay RI associated with the supervisory lamp LI I2, and a dial relay RI20 associated with the dial key KI04 and with the dial DI08.

Also as shown in Fig. 1, the trunk circuit I00'IC terminates in a jack J I5I at the operator switchboard I00 and comprises a line lamp LI52, a busy lamp LI53 and a relay group including a sleeve relay RI60, a jack relay RITE! and a signal-in relay RI80. The trunk circuit I 00TC is connected to the coupling circuit 200 by two line conductors CIQI and CI92, a signal-out conductor CI93 and a signal-in conductor CI54 as explained more fully hereinafter.

As shown in Fig. 4., the trunk circuit 400TC terminates in an outgoing trunk 5I0 accessible to the automatic switching apparatus 500 and an incoming trunk 520 having access to the automatic switching apparatus 500 as previously noted; and comprises a, relay group including a hold relay R4I0, a hold relay R420, a timer relay R430, a dial relay R440, a timer relay R450, a tone relay R460, an answer relay R465, a switch relay R410, a signal-in relay R480, a supervisory relay R435, .a line relay R490 and a reverse battery relay R495. The trunk circuit 400TC is connected to the coupling circuit 300 by two line conductors 040i and C402, a signal-out conductor C403 and a signal-in conductor C404 as explained more fully hereinafter.

As shown in Fig. 5, the subscriber substation T has a. subscriber line 5353 extending thereto, with which a line circuit 535 is individually associated. The automatic switching apparatus 500 comprises a plurality of groups of finder-selector links, each provided with an associated distributor, and corresponding to the groups of subscriber substations. One of the groups of finder-selector links includes the finder '542 and the selector MI and is provided with the individually associated distributor 540; which group of finder-selector links is associated with the group of subscriber substations, including the subscriber substation T. Also, the automatic switching apparatus 500 comprises a plurality of groups of connectors respectively associated with the groups of subscriber substations. One of the groups of connectors includes the connector 510 and is associated with the group of subscriber substations. including the subscriber substation T. Also, the automatic switching apparatus 500 comprises a group of outgoing selectors, including the outgoing selector 550, and an incoming selector 560. More particularly, the finder 542 of the finder-selector link illustrated has access to the subscriber line 530 extending to the subscriber substation '1; while the selector 54I of the finder-selector link illustrated has access to the connector 510 by Way of the associated trunk 51 I, and has access to the outgoing selector 550 by way of the associated trunk 55L Also, the outgoing selector 550 has access to the outgoing trunk 5I0 extending into the trunk circuit 400TC; while the incoming trunk 52H extending from the trunk circuit 400TC is terminated in the incoming selector 560. Finally, the incoming selector 560 has access to the confinder 5% comprises a plurality of sets of post normal switch springs, one of which is indi cated at 856%, which sets of switch springs are actuated selectively in response to operation of the finder 542 different numbers of steps in the vertical direction in the selection of the corresponding ten line groups; and which sets of switch springs are connected to individually associated class tone conductors. Specifically, the set of switch springs 85% is actuated into closed circuit relation in order to connect the associated class tone conductor C546 to the associated condenser 545 when the finder 542 is operated a predetermined number of steps in the vertical direction to select the ten line group of subscriber lines, ineluding thesubscriber line 53!], extending to the subscriber substation T illustrated. Thus it will be understood that the subscriber substation T simplex of the secondary winding of the repeating coil 3M and, consequently, to the simplex of the trunk 3055. Also the hybrid coil 30! is included in an incoming channel provided with an audio is in a given ten line group rendered a particular 7 class of service, as indicated by the signal present on the class tone conductor C545.

Similarly; the selector 54! comprises "a set of post normal switch springs $543 which is operated into closed circuit relation when the selector 54! is operated to a predetermined level to select the trunk 55! extending to the outgoing selector550 and indicative of a trunkcall from the automatic exchange 2!} to the manual exchange IE.

As shown inFig. 2, the coupling circuit 209 in the manual exchange it comprises a hybrid coil terminating the line conductors CHM and C! 92 extending to the trunk circuit llliJTC and provided with a balancing network 2B2. Also the hybrid coil 2M is included in an outgoing channel provided with an audio filter 2st and terminating in the primary winding of a repeating coil 204; the second winding of the repeating coil 201i terminating the truni: 2&5 extending from the coupling circuit 'Zilfl in the manual exchange it to the coupling circuit 236 in the radio station 225W. The signal-out conductor CW3 extending from the trunk circuit IllflTC is connected to the sim plex of the secondary winding ofthe repeating coil 25% and, consequentl to the simplex of the trunk 2%. Also the hybrid coil 20! is included in an incoming channel provided with an audio filter 255 and terminating in the secondary winding of a repeating coil 254; the primary winding of the repeating coil 2% terminating the trunk 253 extending from the coupling circuit 23H in the radio station 225W to the coupling circuit 209 in the manual exchange ii).

ductorcisd extending to the trunk circuit HBQTC is connected to the simplex of the primary winding of the repeating coil 26 and, consequently, to the simplex of the trunk 263.

As shown in Fig. 3, the coupling circuit 300 in the automatic exchange 26 comprises a hybrid coil 3%! terminating the line conductors C401 and C502 extending to the trunk circuit fiiflilTC and provided with a balancing network 302. Also the hybrid coil 30! is included in an outgoing channel provided with an audio filter 303 and terminating in the primary winding of a repeating coil 304; the secondary winding of the repeating lcoiltfid The signal-in con'- terminating the trunk 355 extending from the coupling circuit 356 in the automatic exchange 29 to the coupling circuit 338 in'the radio station 325E. The signal-out conductor 0M3 extending from thetrunk circuit GOUTC is connected to the filter 365 and terminating in the secondary winding of a repeating coil 364; the primary winding I of the repeating coil 3B4 terminating the trunk 363 extending from the coupling circuit 330 in the radio station 3251] to the coupling circuit 30!] in the automatic exchange 20. The signal-in conductor CAM extending to the trunk circuit i-QUTC is connected to the simplex of the primary winding of the repeating coil 36d and, consequently, to the simplex of the trunk 353.

As shown in Fig. 2, the coupling circuit 23!) in the radio station 225W comprises a repeating coil 206, the primary winding of which terminates the trunk 205 extending from the coupling circuit 295 in the manual exchange III; the secondary winding of the repeating coil 2% extending to a transmitting low pass filter 20? that, in turn, is connected to the modulating circuit of the F M. radio transmitter 2H1. Also the modulating circuit of the F. M. radio transmitter 240 is connected by way of a transmitting high pass filter 233 to a transmitting amplifier 232 which, in turn, is connected to a signal oscillator 23!. Also the coupling circuit 23%) is provided with a signal-out relay R215 connected to the simplex of the pri mary winding of the repeating coil 206 and, con sequently, to the simplex of the trunk 255 extending from the coupling circuit 260 in the manual exchange I0; which signal-out relay R2! 5 is pro vided with contacts Zlfi arranged in the plate circuit of the transmitting amplifier 232. Also the coupling circuit 235 comprises a hold relay R220 that is operated by the signal-out relay R2I5 and provided with contacts 229 included in the start circuit of the F. M. radio transmitter 2W, The transmitting low pass filter 28! is arranged to couple audio frequency from the trunk 2135 to the modulating circuit of the F. M. radio transmitter 2!!) and to block the passage of intermediate frequency therethrough from the modulating circuit of the F. M. radio transmitter 2!!) back to the trunk 2315. The signal oscillator 23! is arranged to produce an intermediate 4.3 kc. signal which signal may be amplified by the transmitting amplifier 232 and impressed through the transmitting high pass filter 233 upon the modulating circuit of the F. M. radio transmitter 250s Also the'transmitting high pass filter 233 blocks the passage therethrough of audio frequency from the modulating circuit of the F, M. radio transmitter 2H1 back into the transmitting amplifier 232. Y

Further, the coupling circuit 230 comprises a ground potential to the simplex of the secondargv winding of the rep-eater'coil262 and, consequent ly, to the simplex of the trunk 263 extending to the coupling circuit 260 in the manual exchange Ill. Also the coupling circuit 23!; comprises a receiving high pass filter 24! connected between the output circuit of the F. M. radio receiver 2 and a receiving amplifierZ iZ; the output of the receiving amplifier 242 being connected directly to a:rectifier 243 that, in turn, is connectedito .a D. C. amplifier 244. The output of the D. C. amplifier 244 is connected to the winding of the signal-in relay R245. The D. C. amplifier 244 normally supplies sufiicient current to the winding of the signal-in relay R245 to maintain the latter relay in its operated position; whereby the signal-in relay R245 maintains interrupted, at is contacts 246, the previously menti'nrd path for applying ground potential to the simplex of the secondary winding of the repeating coil 262 and, consequently, to the simplex of the trunk 263. The receiving low'pass filter 25! is arranged to couple audio frequency from th output circuit of the F. M. radio receiver 240 to the trunk .263

and to block the passage .of intermediate :frequency therethrough from the output circuit .of the F. .M. radio receiver 240 to the trunk 203. The receiving high pass filter 2 1-! is arranged .to pass the intermediate 4.5 kc. signal from the output circuit of the F. M. radio receiver 240 to the receiving amplifier 242 and to-block the .passage therethrough of audio frequency from the output circuit of the F. M. radio receiver 240 to the receiving amplifier 242. The intermediate 4.5 kc. signal passed into the receiving amplifier 242 is amplified and passed into the rectifier.243; wherein it is rectified and passed into the D. C. amplifier 244. When the rectified signal is .thus passed into the D. C. amplifier 244, the biasin the D. C. amplifier 244 is increased in order to cause the output current therefrom to the winding of the signal-in relay R245 to be reduced suiiiciently to bring about the restoration of .the signal-in relay R245. Upon restoring the signalin relay R245 completes, at th contacts 246, the previously mentioned path for applying ground potential to the simplex of the secondary. winding of the repeating coil 262 and, consequently,

to the simplex of the trunk 263.

The F. M. radio receiver240 comprises among other apparatus, a detector stage, an output stage and a squelch circuit coupling the detector stage to the output stage; whereby the sensitivity of the output stage is greatly reduced when the output of the detector stage does not include either the 4.5 kc. or an audio component. .On the other hand, when the output of the detector stage includes either the 4.5 kc. or an audio component, the sensitivity of the output stage is greatly increased. This squelch circuit arrangement prevents theF. M. radio receiver 240 from being noisy when a radio frequency carrier signal is being received that is not being modulated by the 4.5 kc. or an audio signal. Also the coupling circuit 230 comprises a start relay R250 included i in the squelch circuit of the F. M; radio receiver 240; which start relay R250is operated when the squelch circuit is operated to increase the sensitivity of the output stage of the F. M. radio 'receiver 240. The start relay R250 is provided with contacts 25! included in the start circuit of the F. M. radio transmitter 2l0; whereby operation of the F. M. radio transmitter 2l0 is initiated, in the event it is restored, when'the F. M.'r-adio receiver 240 receives the radio frequency carrier modulated by either the 45 kc. or an audio signal.

Finally, it is noted that the F. M. radiotransmitter 210 transmits a frequency of 35.42 mc.; which radio frequency carrier may be modulated both by audio frequency signals received over the trunk 205 from the coupling circuit 200 and by the 4.3 kc. signal from the intermediate frequency generator or signal oscillator 23!. Simi- 'larly, the F. Mr radio receiver 240-receives:at"a

frequency of 33.18 mc.; which radio frequency carrier may be modulated both by .audio frequency signals and by the intermediate 4.5 kc. signal.

As shown in Fig. 3, the coupling circuit 330 in the radio station 325E comprises a repeating coil 306, the primary winding of which terminatesthe trunk 305 extending from the coupling circuit 300 in the automatic exchange 20; the secondary winding of the repeating coil 306 extending to a transmitting low pass filter 301 that, in turn, is connected to the modulating circuit of the F. M. radio transmitter 310. Also the modulating circuit of the F. M. radio transmitter 3) is connected by way of a transmitting high pass filter 333 to a transmitting amplifier 33.2 which, in turn, is connected to a signal oscillator 33!. Also the coupling circuit 330 is provided with a signal-out relay R3I5 connected to the simplex of the primary winding of the repeating coil 306 and, consequently, to the simplex of the trunk 305 extending from the coupling circuit 300 in the automatic exchange 20; which signal-out relay R3l5 is provided with contacts 316 arranged in the plate circuit of the transmitting amplifier 332. Also the coupling circuit 330 comprises a hold relay R320 that is operated by the signal-out relay R3l5 and provided with contacts 32I included in the start circuit of the F. M. radio transmitter 3l0. The transmitting low pass filter 30'! is arranged to couple audio frequenc from the trunk 305 to the modulating circuit of the F. M. radio transmitter M0 and to block the passage of intermediate frequency therethrough from the modulating circuit of the F. M. radio transmitter 3l0 back to the trunk 305. The signal oscillator 33l is arranged to produce an intermediate 4.5 kc. signal which signal may be amplified by the transmitting amplifier 332 and impressed through the transmitting high pass filter 333 upon the modulating circuit of the F. M. radio transmitter 310. Also the transmittin high pass filter 333 blocks the passage therethrough of audio frequency from the modulating circuit of the F. M. radio transmitter 3| 0 back into the transmitting amplifier 332.

Further, the coupling circuit 330 comprises a repeating coil 362, the secondary winding of which terminates the trunk 363 extending to the coupling circuit 300 in the automatic exchange 20; the primary winding of the repeating coil 362 being connected by way of a receiving low pass filter 36l to the output circuit of the F. M. radio receiver 340. Also the coupling circuit 330 comprises a signal-in-relay R345 provided with contacts 346 that are adapted when closed to connect ground potential to the simplex of the secondary winding of the repeating coil 362 and, consequently, to the simplex of the trunk 363 extending to the couplin circuit 300 in the automatic exchange'20. Also the coupling circuit 330 comprises a receiving high pass filter 341 connected between the output circuit of the F. M. radio receiver 340 and a, receiving amplifier 342; the output of the receiving amplifier 342 being connected directl to a rectifier 343 that, in turn. is connected to a D. C. amplifier 344. The output of the D. C. amplifier 344 is connected to the winding of the signal-in relay R345. The D. C. amplifier 344 normally supplies suiiicient current to the winding of the signal-in relay R345 to maintain the latter relay in its operated position;

whereby the signal-in relay R345 maintains interrupted, .at its contacts 346, the previously saga- 14 iiientioned path for applying ground potential to the simplex of the secondary winding of the repeating coil 362 and, consequently, to the simplex of the trunk 363. The receiving low pass filter 36! is arranged to couple audio frequency from the output circuit of the F. M. radio receiver 340 to the trunk 363 and to block the passage of in termediate frequencytherethrough from the output circuit of the F. M. radio receiver 340 to the trunk 363. The receiving high pass filter 34l is arranged to pass the intermediate 4.3 kc. signal from the output circuit of the F. M. radio receiver 340 to the receiving amplifier 342 and to block the passage therethrough of audio frequency from the output circuit of the F. M. radio receiver 349 to the receiving amplifier 342. The intermediate 4.3 kc. signal passed into the receiving amplifier 342 is amplified and passed into the rectifier 343; wherein it is rectified and passed into the D. C. amplifier 344. When the rectified signal is thus passed into the D. C. amplifier 344, the bias in the D. C. amplifier 344 is increased in order to cause the output current therefrom to the winding of the signal-in relay R345 to be reduced sufiiciently to bring about the restoration of the signal-in relay R345. Upon restoring the signal-in relay R345 completes, at the contacts 345, the previously mentioned path for applying ground potential to the simplex of the secondary winding of the repeating coil 362 and, consequently, to the simplex of the trunk 363.

The F. M. radio receiver 340 comprises among other apparatus, a detector stage, an output stage and a squelch circuit coupling the detector stage to the output stage; whereby the sensitivity of the output stage is greatly reduced when the output of the detector stage does not include either the 4.3 kc. or an audio component, On the other hand, when the output of the detector stage includes either the 4.3 kc. or an audio component, the sensitivity of the output stage is greatly increased. This squelch circuit arrangement prevents the F. M. radio receiver 349 from bein noisy when a radio frequency carrier signal is being received that is not being modulated by the 4.3 kc. or an audio signal.. Also, the coupling circuit 330 comprises a, start relay R3511 included in the squelch circuit of the F. M. radio receiver 343; which start relay R350 is operated when the squelch circuit is operated to increase the sensitivity of the output stage 'of the F. M. radio receiver 340. The start relay R35!) is provided with contacts 35! included in the'start circuit of the F. M. radio transmitter 310; whereby operation of the F. M. radio transmitter 3 I is initiated, in the event it is restored, when the F. M. radio receiver 340 receives the radio frequency carrier modulated by either the 4.3 kc. or an audio signal.

Finally, it is noted that the F. M. radio transxn1tter 3l9, transmits ata frequency of 33.18 me; which radio frequency-carrier ma be modulated both by audio frequencysignals received over the trunk 385 from the coupling circuit 300 and by the 4.5 kc. signal from the intermediate frequenc generator or signal oscillator 33!. Similarly, the F. M. radio receiver 340 receives at a frequency of 35.42 meg-which radio frequency carrier may be modulated both by audio frequency signals and by the intermediate 4.3 kc. signal. i A

A better understanding of the connection and arrangement of the apparatus incorporated in theradiotelephone system illustrated in Fig.6 and briefly described above will be. facilitated from a consideration of the; extension of connec- '10 tions involving the manual exchange l9 and the automatic exchange 20.

Considering now the extension of a connection between the manual exchange it and the automatic exchange 28, and assuming that a call has been extended from a callingsubscriber substation, not shown, in the manual exchange IS to the operator switchboard its therein and that the operator at the switchboard has answered the call utilizing the cord circuit at this time the plug Pltl3 of the cord circuit iii has been inserted into the jack terminating the subscriber line extending to the calling subscriber substation and the talk kay Klll5 has been operated, in order to connect the operator headset M3 to the cord circuit lIJl. At this time the operator at the switchboard Hi9 learns that the subscriber at the calling subscriber substation in the manual exchange it desires to convers with a subscriber in the automatic exchange 2&3, such, for example, as the subscriber at the called subscriber substation '1. The operator at the switchboard Hi3, observing the condition of the trunk circuit illilTC, as indicated by the unlighted bus lamp Libt, selectsthe trunk circuit mentioned and inserts the plug PI32 of the cord circuit ifii into the terminating jack J l5l, assuming that the trunk circuit ltiilTC is idle at this time.

When the plug P102 is inserted into the jack J the contacts l5! thereof are operated, com-- pleting an obvious circuit for operating the jack relay Rilii in the trunk circuit EBBTC. Also, a series circuit, including the contacts E24, is completed for energizing the supervisory relay R530 in the cord circuit Ifli and the lower winding of the sleeve relay EH30 in the trunk circuit iiitiTC, whereby the sleeve relay R433 operates. However, the supervisory relay Rl3il does not operate as it is ofthe marginal type. Upon operating, the jack relay Rilll completes, at the contacts I73, an obvious circuit'for illuminating the busy lamp Ll 53, thereby to indicate the busy condition of the trunk circuit IMTC; and interrupts at the contacts H5, an obvious path for short-circuiting the upper winding of the signal-in relay EH33 in order. to increase the speedoi response of the latter relay. Also, the jack relay Rllil completes,

at the contacts H2, a circuiuincluding the con tacts M1, for energizing the upper winding of the sleeve relay Rite in multiple with the lower winding thereof and in series with the winding of the supervisory relay RE 33, whereb the supervisory relay R1311 operates. Upon operatingfthe supervisory relayRi3ii completes, at the contacts I3 I, an obvious circuit for illuminating the supervisory lamp Ll l l in the cord circuit iiii. Finally,

upon operating, the jack relay Rl'lc completes, the contacts ill, a circuit, including the contacts 163 and its, for applying battery potential by way of the resistor 168 to the signal-out conductor C193, assuming that the dial key Kits in the cord circuit lfll is operated at this time; which application ofresistance battery potential to the signal-out conductor C593 completes a circuit for operating the signal-out relay REES in the coupling circuit 233 at theradio station 225W. The above mentioned circuit extends from battery by way of the resistor $33, the contacts [66, the ring of the jack Jitil, the ring of the plug PHIZ, the impulse contacts lie of the dial D! 08, the upper contacts of the dial key K'IM, the tip of the plug Pitt, the tip of the jack Jlii i, the contacts I63 and ill, the signal-out conductor (H33, the simplex of the secondary winding of the repeating coil 204, the trunk 28%, the simplex 11 of the primary winding of the repeating coil 206 and the winding of the signal-out relay R2I5 to ground potential. Upon operating, the sleeve relay RI60 completes, at the contacts I6I, a multiple circuit, including the contacts I'II, for applying battery potential by way of the resistor I67 to the signal-out conductor CI93; and then interrupts, at the contacts I63 and IE6, the above-traced original path for applying battery potential by way of the resistor I68 to the signalout conductor CI93. Also, the sleeve rela RI60 interrupts, at the contacts I64, a normally completed termination circuit, including the resistor I69, between the line conductors CI9I and CI92 extending between the trunk circuit IOTC and the hybrid coil 20I in the coupling circuit 200. Finally, the sleeve relay RIBI] completes, at the contacts I62 and I65, a connection between the line conductors CI9I and CI92 and the tip and ring of the jack J II and, consequently, to the tip and ring of the plug PI02, which at this time are operatively connected together by wa of the upper contacts of the dial key KIM and the impulse contacts E0 of the dial IDI08.

The operator at the switchboard I00 then proceeds to dial the first digit of the directory number of the called subscriber substation T in the automatic exchange 20. When the dial DI08 is thus operated the off-normal springs I09 thereof are first actuated into engagement, thereby to complete a circuit, including the lower contacts of the dial key KIM, for operating the dial relay RI in the cord circuit IOI. Upon operating, the dial relay RI20 interrupts, at the contacts I2I, the previously traced series circuit for energizing the winding of the supervisory relay RI30 in the cord circuit WI and the upper and lower multiply connected windings of the sleeve relay RI60 in the trunk circuit I00'I'C, whereby the relays mentioned restore. Upon restoring, the supervisory relay RI30 interrupts, at the contacts I3I, the previously mentioned circuit for illuminating the supervisory lamp L! I I in the cord circuit NH; and, upon restoring, the sleeve relay RI60 recompletes, at the contacts I63 and I66, the previously traced original path for applying battery potential by way of the resistor I68 to the signal-out conductor CI93, which path includes the upper contacts of the dial key KIM and the impulse contacts IIEl of the dial DI08, both inthe cord circuit I0 I, as previously noted. Further, the sleeve relay RI60 interrupts, at the contacts I6I, the previously traced alternative path for applying battery potential by way of the resistor I61 to the signal-out conductor CI93; interrupts,

at the contacts I62 and I65, the previously traced path, including the upper contacts of the dial key KIO and the impulse contacts H0 of the dial DI08 in the cord circuit IEII, across the line conductors CI9I and CI62; and recompletes, at the contacts I64, the previously mentioned termination circuit including the resistor I69 between the line conductors CHM and CI92. At this time battery potential is applied by way of the resistor I68 to the signal-out conductor CI93 over the above-traced path including the impulse contacts H0 of the dial DI08 in the cord circuit IIlI. Accordingly, as the dial DI08 is returned to its normal position the impulse contacts Hi) thereof are operated one or more times, corresponding to the number of impulses in the digit dialed, effecting interruption and then recompletion a corresponding number of times of the above-traced path for applying battery potential by way of the resistor I68 to the signallit out conductor 'CI93, whereby the signal-out relay R2I5 in the coupling circuit-260 follows the impulses of the present digit. At the conclusion of the first digit when the dial DI08 in the cord circuit IOI is returned to its normal position, the impulse contacts IIO thereof are maintained in closed circuit position and the off-normal con- "tacts I09 thereof are actutaed into disengagement in order to interrupt the circuit for maintaining operated the dial relay RI20 in the cord circuit IBI. Shortly thereafter the dial relay RIZI] restores, as it is of the slow-to-release type, recompleting, at the contacts I2I, the previously traced series circuit for energizing the winding of the supervisory relay Ri30 in the cord circuit WI and the multiply connected upper and lower windings of the sleeve relay RI60 in the trunk circuit IO0TC, causing the relays mentioned to reoperate. Upon reoperating, the supervisory relay RI3B again completes, at the contacts I3I, the previously mentioned circuit for illuminating the supervisory lamp LIII; and, upon reoperating, the sleeve relay RI60 recompletes, at the contacts H, the previously traced alternative path for applying battery potential by way of the resistor I61 to the signal-out conductor CI93; and interrupts, at the contacts I63 and I66, the previously traced original path for applying battery potential by way of the resistor I68 to the signal-out conductor CI93. In a similar manner the operator at the switchboard I 60 dials the second and third digits of the directory number of the called subscriber substation T in the automatic exchange 20 and then, at the conclusion of the dialing of the third digit mentioned, she restores the dial key KIM to its normal position, thereby interrupting the previously traced bridge including the impulse contacts H6 of the dial DI08 between the tip and the ring of the plug PI02 of the cord circuit IOI. At this time the .tip and the ring of the plug PIElZ are operatively connected by way of the condensers I32 and I33 to the tip and the ring of the plug PI03 and, consequently, by way of the contacts of the talk key KI'05 to the operator headset I I3.

Inview of the foregoing explanation of the manner in which the operator at the switchboard I00 seizes the trunk circuit i dilTC utilizing the cord circuit I01, it will be understood that resistance battery potential is first steadily ap- CI'QS. Accordingly, the signal-out relay REL; in

the coupling circuit 230 operates steadily; then follows the impulses of the three spaced-apart digits mentioned and then operates steadily again. Upon operating'the signal-out relay R2 8 5 completes, at the contacts 2II, an obvious circuit for operating the hold relay R226; whereby the latter relay upon operating completes, at the contacts 22I, the start circuit of the F. M. radio transmitter 2I0 in order to cause operation thereof to transmit the 35.42 mc. radio frequency carrier. Also upon operating the signal-out relay R2 I 5 completes, at the contacts 2 t6, the plate circuit of the transmitting amplifier 2212 connected to the signal oscillator 23I, producing the intermediate 4.3 kc. signal; whereby the transmitting amplifier 232 operates to impress the intermediate 4.3 kc. signal through the transmitting highpass filter 233 upon thcmodulating circuit of the F. M. radio transmitter 210 in order to cause the 35.42 mc. radio frequency carrier to be modulated by the intermediate 4.3 kc. signal. Each time the signal-out relay R215 restores and then reoperates, during the reception of a digit, it interrupts and. then recompletes, at the contacts 2I1, the previously mentioned circuit for maintaining operated the hold relay R220; however, Without eifect since the hold relay R 220 being of the sloW-to-release type, remains operated during the reception of. a digit. Also, each time the signal-out relay R215 restores and then reoperates, during the reception of a digit, it interrupts and then recompletes, at the contacts 216, the plate circuit of the transmitting amplifier 232; whereby the transmitting amplifier 232 operates to interrupt and then to project the intermediate 4.3 kc. signal over the transmitting high pass filter 233 to the modulating circuit of the F. M. radio transmitter2l0. Accordingly, the transmitting amplifier 232 transmits amplified pulses of the intermediate 4.3 kc. signal through the transmitting high pass filter 233 to the modulating circuit of the F. M. radio transmitter 210 in accordance with the impulses of the digit being received by the signal-out relay R215. Hence, the F. M. radio transmitter 210 operates to transmit unmodulated pulses of 35.42 mc. radio frequency carrier in accordance with the impulses of the digit being received by the signal-out relay R215.

When operation of the F. M. radio transmitter 210 in the radio station 225W is initiated to transmit the 35.42 mc. radio frequency carrier modulated by the intermediate 4.3 kcxsigrial, the F. M. radio receiver 340 in the radio station 325E receives; whereby the output of the detector stage therein includes the 4.3 kc. component causing the squelch circuit therein to increase the sensitivity of the output stage therein and to operate the start relay R350 in the coupling circuit 335. Upon operating the start relay R350 closes the contacts 35! completing the start circuit of the F. M. radio transmitter 310 in the radio station 325E. At this time the F. M. radio transmitte 310 transmits the unmodulated 33.18 mc. radio frequency carrier, which unmodulated 33.18 mc. radio frequency carrier is received by the radio receiver 240 in the radio station 225W; howeve without effect since the output of the detector stage therein does not include the 4.5 kc. component such that the squelch circuit therein is not operative to increase the sensitivity of the output stage therein.

When the F. M. radio receiver 340 in the radio station 325E first receives the 35.42 mc. radio frequency carrier modulated by the'intermediate 4.3 kc. signal, the 4.3 kc. component appears in the output stage therein and is passed through the receiving high pass filter 345 into the receiving amplifier 342; which 4.3 kc. component is amplified therein and passed directly into the rectifier 343. The signal is rectified by the rectifier 343 and passed into the D. C. amplifie 344; whereby the bias therein is increased causing the D.. C. amplifier 344 to decrease its output to the winding of .the signal-in relay R345 suificiently to cause the latter relay to restore. Upon restoring, the signal-in relay R345 completes, at the contacts 346, a path including the simplex of the secondary winding of the repeating coil 362, the trunk 363 and the simplex of the primary winding of the repeating coil 364 for applying ground; potential to the signal-in conductor C404 34 extending from the coupling circuit 301 to the trunk circuit 400TC in the automatic exchange 20. When the radio receiver 340 in the radio station 325E then receives the unmodulated pulses of 35.42 mc. radio frequency carrier corresponding to the impulses of the first digit, the 4.3 kc; component is alternately absent and present-in the output stage thereof; whereby pulses of the 4.3 kc. component are passed through the receiving high pass filter 341 into the receiving amplifier 342 and amplified and passed to the rectifie 343. The pulses of signal from the rectifier 343 are passed into the D. C. amplifier 344 in order alternately to increase the bias therein; whereby corresponding pulses of current pass to the winding of the signal-in relay R345 causing the latter relay to follow. In view of the foregoing, it will be understood that the signal-in relay R345 in the coupling circuit 333 restores steadily; then follows the impulses of the three spaced-apart digits mentioned; and then re stores steadily again. Also during the reception of a digit by the F. M. radio receiver 349 in the radio station 325E, each time the 4.3 kc. com ponent is absent from the output of the detector stage therein, the squelch circuit operates to decrease the sensitivity of the output stage therein and to de-energize the winding of the start relay R350; however without effect since the start relay R350 being of the slow-to-release type, remains operated during the reception of a digit.

Upon first restoring, the signal-in relay R345 in the coupling circuit 330 completes, at the contacts 346, the previously traced path for applying ground potential by way of the simplex of the trunk 353 to the signal-in conductor C404 extending into the trunk circuit 450TC in the automatic exchange 20; which application of ground potential to the signal-in conductor C404 completes an obvious circuit for operating the signal-in relay R480 in the trunk circuit illilTC. Accordingly, the signal-in relay R430 operates steadily; then follows the impulses of the three spaced-apart digits mentioned; and then operates steadily again.

Upon first operating, the signal-in relay R480 completes, at the contacts 48!, an obvious path for short-circuiting the resistor 421 and the series connected condenser 425; and completes, at the contacts 483, an obvious circuit for operating the hold relay R420. Upon operating, the hold relay R420 completes at the contacts 424', .a. circuit, including the contacts M6, for energizing the lower winding of the switch relay R410, thereby to cause the latter relay to operate. Upon operating, the switch relay R410 interrupts, at the contacts 411 and 414, the normal connections between the line conductors C4? and C402 extending from the hybrid coil 30! in the coupling circuit 300 and the line conductors C5 .and C5I2 for the outgoing trunk 51!] extending to the trunk circuit 4'00TC and accessible to the outgoing selector 550. Also, the switch relay R410 completes, at the contacts 412, 413 and 415, a connection between the line conductors C521 and C522 of the incoming trunk' mam IS the seizure of the incoming selector, 560; where.- by it is. rendered responsive to the first digit transmitted from the operator switchboard I00. Also the switch relay R470 interrupts, at. the contacts 411, a normally completed. path including the contacts 43: for applying battery potential by way of theimpedance 428 to the controlconductor C5l3 of the outgoingtrunk 5|0; and completes, at the contacts 418; a substantially identical path, including the contacts M3, for applying ground potential to, the control conductor C5l3 of the outgoing trunk 510. When battery potential is thus removed from and ground potential is thus applied to the control conductor C5l3 of the outgoing trunk 5l0', the marking of the outgoing trunk 5l0 as idle is removed and the marking of the outgoing trunk 5l0' as busy is brought about. Accordingly, theoutgoing trunk 5l0, and consequently the trunk circuit 400TC', is marked as busy to the outgoing selectors 550, etc. at this time. When the incoming selector 560 is conditioned to be responsive to the first digit transmitted from the operator switchboard I00 as above noted, ground potential therein is returned over the control'conductor C523 of the incoming trunk- 520, thereby to complete a circuit for energizing the left-hand winding of the answer relay R465 .and a multiple holding circuit for'energizing the upper winding of the switch relay R410; The answer rela R465 doesnot operate at this time, since itis oithe shunt field type, and will not operate until the right-hand winding thereof is energized in the opposite direction over the, loop circuit extendingby way of the line conductors Q52] and C522 of the incoming trunk520 to the incoming selector 560. At this time the radio link has seized the trunk circuit 400TC and the latter trunk circuit has seized the incoming selector 560 rendering it responsive to the first digit transmitted from the operator switchboard I00.

Whenthe first digit is received over the radio link, the signal-in relay R345 in the coupling circuit 330 in the radio station 325E follows repeating, at the contacts 346, the impulses thereof to the signal-in relay R480 in the trunk circuit 4l10TC; whereby the latterrelay follows as previously explained. Each time the signal-in rela R488 restores and then reoperates it interrupts and then recompletes, at, the contacts 483, the previously mentione circuit for maintaining operated the hold relay R420, thereby to.cause the latter relay to remain operated during impulsing since it is of the slow-to-release type. Also, each time the signal-in relay R480 restores and then reoperates it completes and then interrupts, at the contacts 48,2, a, circuit, including the contacts. 422, for operating the, dialrelay R440, thereby, tocause the latter relay to operate and remain operated-during impulsing as it is of the slow-to-release type. Upon operating, the dial relay R440 completes, at the contacts 4.4!, apath, including the resistor M9, in multiple with the right-hand winding of the an- I swer relay R465, across the-line conductors 052i and C522 of the incoming trunk 520, thereby to improve impulsing to the incoming selector= 560; Finally, each time the signal-in relay'R480 restores and reoperates it interrupts and then recompletes, at the contacts 48l, the previously traced loop circuit extending by way of the line conductors C521 and C522 of the incoming trunk 520, whereby the impulses of thefirst digit are transmitted to, the incoming selector 560 in order to cause, corresponding operation, thereof. More particularly, the incoming selector 560 operatesin response to the first digit to selectthe, corresponding group of. connectors, including the connector 510, and then operates automatically to select an idle connector in the previously se, lected group. For example, the incoming selector 560 may. operate to seize the trunk 5' ex: tending to the connector 570, whereby the con-v nector 510 is conditioned to be responsive to the. second and third digits transmitted from the operator switchboard I50. At the conclusion ofv the first digit transmitted from the operator switchboard I00, the signal-out relay R215 in the coupling circuit 230. in the radio station-225W' is retained in its operated position; thereby to cause the signalin relay R345 in the coupling circuit 330. in the radio station 32513 to beretained A its restored position; whereby thesignal-in' relay R480 in the trunk circuit 400TC is retained in its operated position. The operated signal-in relay R480 retains completed the previously mentioned circuit for maintaining operated the hold relay R420; and retains completed the previously traced loop circuit extending by way of the line conductors 052i and C522 of the incoming trunk 520 to the incoming selector 560. Finally, the operated signalin relay R480 maintains interrupted, at the contacts 482, the previously traced-circuit for maintaining operated the dial relay R440, whereby the latter relay restores shortly thereafter as it is of the slow-to-release type. Upon restoring, the dial relay R440 interrupts, at the contacts 44 l, the previously mentioned path, including the resistor M9, for short-circuiting the right-hand Winding of the answer relay R465.

The second and third digits transmitted from the operatorswitchboard I00 over the radio link are repeated to the trunk circuit 40UTC in an identical manner; whereby the second and third digits mentioned arerepeated by thesignal-in relay R480 in the trunk circuit 400TC over the incoming trunk- 520-, the incoming selector 560 and the trunk 5H to the connector 510 in order to cause corresponding operation thereof to select the subscriber line 530 extending to the called subscriber substation T in the automatic exchange 20; whereby the idle or busy condition thereof is testedin the connector 510. First assuming that the subscriber substation T is idle at this time, battery potential appears upon the control conductor C534 of the subscriberline 530 causing the connector 510 to operate in order to seize the subscriber line 530, whereby ground potential in the connector 510 is applied to the control conductor C534 of the subscriber line 530 extending to the associated line circuit 535, thereby marking the subscriber substation T as busy to the other connectors in the group, including the connector 510' having access thereto. Also, the connector 510 operates to project ringing current over the line conductors C53l and C532 of the subscriber line 530 in order to operate the ringer at the called subscriber substation T and to return ring-back tone current over the line conductors of the trunk 5-H and; consequently, over the incoming selector 56 0, the line conductors C52! and C522 of the incoming trunk 520, and the line conductorsv C402 and C40I to the hybrid coil 30| in the coupling circuit 300. Theringback tone current is passed from the hybrid coil 30l through the audio filter 303 into the. primary winding of the repeating; coil 304; whereby a .corresponding; signal induced in the secondary wind:-

ing of the repeating coil 3M passes over the trunk 305 into the primary winding of the repeating coil 306 in the coupling circuit 335 in the radio station 325E. A corresponding signal is induced in the secondary winding of the repeating 0011505 and is impressed through the transmitting. low pass filter 30'! upon the modulating circuit of the F. M. radio transmitter 3H], which is operating at this time since the start circuit for the F. M. radio transmitter 3!!! is closed at the contacts 35! by the operated start relay R555. Accordingly, the F. M. radio transmitter 3H3 modulates the 33.18 mc. radio frequency carrier in accordance with the audio ring-back tone signal mentioned.

The audio ring-back tone signal modulated 33.18 mc. radio frequency carrier is received by the F. M. radio receiver zen in the radio station 225W; whereby the audio ring-back tone component in the output of the detector stage therein causes the squelch circuit therein to increase the sensitivity of the output circuit therein; whereby the audio ring-back tone component from the output stage in the F. M. radio receiver 2&5 is passed through the receiving low pass filter 25! into the primary winding of the repeating coil 252. A corresponding signal is induced in the secondary winding of the repeating coil 262 and projected over the trunk 2653 into the primary winding of the repeating coil 25 in the coupling circuit 255. A corresponding audio ring-back tone signal is induced in the secondary winding of the repeating coil 255 and passes through the audio filter 265 into the hybrid coil 20! and thence is applied to the line conductors C 9! and C 92 and by way of the previously traced path to the operator headset! !3 at the switchboard H36 in order to indicate to the operator thereat that the called subscriber substation T in the automatic exchange 253 is being rung.

Also the audio ring-back tone component in the output of the detector stage in the F. M. radio receiver 245 controls operation of the squelch circuit therein to effect operation of the start relay R255 in the coupling circuit 235 in the radio station 225W. Upon operating the start relay R255 completes, at the contacts 25!, a point in multiple to the contacts 22! in the start circuit of the F. M. radio transmitter 2H3.

When the subscriber at the called subscriber substation T in the automatic exchange 20 answers the call, the connector 515 switches through to interrupt the projecton of ringing current over the subscriber line 53!) extending to the called subscriber substation T and the return of ringback tone current over the trunk 51! and thepreviously traced channel, including the radio link, to the operator headset H3 at the switchboard !!lil in the manual exchange Iii. Also, the connector 5753 operates to bring about the reversal of polarity over the line conductors of the trunk 51! and, consequently, over the incoming selector 5% and the line conductors C52! and C522 of the incoming trunk 52!l,whereby the righthand winding of the answer relay 3 355 in the trunk circuit ltfiTC is poled to operate. Upon operating, the answer relay R555 interrupts, at the contacts 45 5, a normally completed termination circuit, including the resistor 525, the contacts 49! and 252, and the condenser t i 8 extending between the line conductors C402 and C45! interconnecting the trunk circuit tlliiTC and the hybrid coil 30! in the coupling circuit 305 in order to prepare the channel for conversation between the operator switchboard H in the manual exchange I!) and the called subscriber substation T in the automatic exchange 2!). Also, the answer relay R465 completes, at the contacts 561, an obvious circuit for energizing the lower winding of the supervisory relay R485, thereby to cause the latter relay to operate and complete, at the contacts 581, an obvious path for applying battery potential by way of the resistor 525 to the signalout conductor C403 extending to the coupling circuit 35E]; thereby to complete a circuit including the simplex of the secondary winding of the reeating coil 355, the trunk 305 and the simplex of the primary winding of the repeating coil 306 for operating the signal-out relay R3l5 in the coupling circuit 335 in the radio station 325E. Upon operating the signal-out relay R3l5 completes, at the contacts 3H, an obvious circuit for operating the hold relay R325; whereby the latter relay upon operating completes, at the contacts 32!, a point in multiple to the contacts 35! in the start circuit of the F. M. radio transmitter 350. Also the signal-out relay R355 completes, at the contacts EH6, the plate circuit of the transmitting amplifier 332 connected to the signal oscillator 33!, producing the intermediate 4.5 kc. signal; whereby the transmitting amplifier 332 operates to impress the intermediate 4.5 kc. signal through the transmitting high pass filter 333 upon the modulating circuit of the F. M. radio transmitter Sill in order to cause the 33.18 mc. radio frequency carrier to be modulated by the intermediate 4.5 kc. signal.

The 33.18 mo. radio frequency carrier modulated by the intermediate 4.5 kc. signal is received by the F. M. radio receiver 245 in the radio station 225W; whereby the output of the detector stage therein includes the 4.5 kc. component causing the squelch circuit therein to increase the sensitivity of the output stage therein and to operate the start relay R258 in the coupling circuit 23B. Upon operating the start relay R250 completes, at the contacts 25!, the previously mentioned point in multiple to the contacts 22! in the start circuit of the F. M. radio transmitter 2H1. The 4.5 kc. component in the output stage of the F. M. radio receiver 245 passes through the receiving high pass filter 26! into the receiving amplifier 242; wherein it is amplified and supplied directly to the rectifier 253. This signal is rectified by the rectifier 2&3 and supplied to the D. C. amplifier 254; whereby the bias therein is increased in order to reduce the current supplied to the winding of the signal-in relay R245 sulficient to bring about restoration of the latter relay. Upon restoring the signal-in relay R245 in the coupling circuit 230 completes, at the contacts 266, a path including the simplex of the secondary winding of the repeating coil 262, the

trunk 263 and the simplex of the primary winding of the repeating coil 255 for applying ground potential to the signal-in conductor Cl l extending between the coupling circuit 2% and the trunk circuit IUUTC. This application of ground potential to the signal-in conductor Clli i completes an obvious circuit for energizing the lower winding of the signal-in rela Rltfl in the trunk circuit IUUTC; whereupon the latter relay operates. Upon operating, the signal-in relay RIB!) completes, at the contacts I82, an obvious multiple circuit for illuminating the busy lamp L! 53; and interrupts, at the contacts !8!, the previously traced circuit for energizing the upper winding of the sleeve relay EH55 in multiple with the lower winding thereof and in series with the winding of the supervisory relay R in. the cord circuit IOI, whereby the sleeve relay R160 is retained in its operated position and the supervisory relay RISU restores as it is of the marginal type. Upon restoring, the supervisory relay Ri30 interrupts, at the contacts I31, the previously mentioned circuit for illuminating the supervisory lamp L! H in the cord circuit Iill, thereby to render to the operator at the switchboard I06 answer supervision. At this time the operator at the switchboard IGO may converse with the subscriber at the called subscriber substation T in the automatic exchange 20 and with the subscriber at the calling subscriber substation in the manual exchange l, since a three-way communication connection has now been established. Then the operator may restore the talk key Kl Bil-of the cord circuit lfii, disconnecting the operator headset 3 from the cord circuit HH, whereby a connection is then completed between the calling subscribed substaticn in the manual exchange if] and the called subscriber substation T in the automatic exchange 26 At this time transmitter battery is supplied, via the upper and lower wind-- ings of the supervisory relay Rita, the back contacts of the ring key K105 and the tip and the ring of the plug P183, to the callingsubscriber substation, whereby the supervisory relay RIM! occupies its operated position maintaining interrupted-at the contacts Ml, a circuit for illuminating the supervisory lamp-LI I'2- in the cord circuit NH. Accordingly, the extinguished supervisory lamp Li [2 and the extinguished supervisory' lamp L! H respectively indicate that the calling subscribersubstation in the manual exchange IE] and the called subscribersubstation T in the automatic exchange 20 are included in the established connection.

At this time audio frequency signals received from the callingsubscriber substation. in the manual exchange Ill-pass through the condensersl32 and I33 in the cord circuit. 161- via the plug PIG-2' and the jack J i! into the trunk circuit IBUTC, and thence over the lineconductors-CHH and Cl92 to the hybridcoil Zill in the coupling circuit 200. The audio frequency'signals then pass from the hybrid coil-201, through. the audio filter 283, through the repeating coil 20.4, over the trunle 2'05, and through. the. repeating coil 206 and the transmitting low pass filter-231 to the modulating circuit of the F. M. radio transmitter Zlfl. Accordingly, at this time-the 35.42 mc. radio frequency carrier is modulatedboth by. the audio frequenc signals and by the.interme-. diate 4.3 kc. signal from the signal oscillator 23L The F. M. radio receiver 340 receives the 35.42: mc. radio frequency carrier. modulated both by the audio frequency signals and the intermediate 4.3 kc. signal. The 4:.3 kc. component passes from the output stage of the F. M. radio receiver 340 through the receiving high pass filter 34,] into the receiving amplifier 342 as previously explained; while the audio frequency signals pass from the output stage of the F. M. radio receiver 340, through the receiving low pass filter 35 I, and; thence through the repeating coil 352 over the trunk 363, and then through the repeating coil 364 and the audio filter 365 into the hybrid coil 3M in the coupling circuit 300. The audio frequency signals then pass from the hybrid coil 36] over the line conductors cam and C402 and, consequently, over the previously traced connection including the trunk circuit lillllTC, the incoming trunk 52? the incoming selector 56%, the trunk 51!, the connector 578, and the subscriber line 53:} to the called subscriber substation in the automatic exchange 20.

ceived from the called subscriber substation T in the automatic exchange 20- pass over the subscriber line 539, the connector 510, the trunk 57 l, the incoming selector 560 and the incoming trunk 52!] to the trunk circuit tfiOTC and thence by way of the line conductors C46! and C 132 to the hybrid coil m the coupling circuit 300. The audio frequency signals then pass from the hybrid coil 33!, through the audio filter 303, through the repeating coil 304, over the trunk 305, and through the repeating coil 33B and the transmitting low pass filter 391 to the modulating circuit of the F. M. radio transmitter em. Accordingly at this time, the 33.18 mc. radio frequency carrier is modulated both by the audio frequency signals and by the intermediate 4.5 kc. signal from the signal oscillator 33L The F. M. radio receiver 2 1E3- receives the 33.18 mc. radio frequency carrier modulated both by the audio frequency signals and the intermediate 4.5 kc. signal. The kc. component passes from the output stage of the F. M. radio receiver 240 through the receiving high pass filter 24! into the receiving amplifier 242 as previously explained; while the audio frequency signals pass from the output stage of the F: M. radio receiver 246, through the receiving low pass filter 26 I, and thence through the repeating coil 262 over the trunk 263, and then through the repeating coil 264 and the audio filter 255 into the hybrid coil 26! in the coupling circuit 200. The audio frequency signals then pass from the hybrid coil 20! over the line conductors CHM and CI92 and, consequently, over the previously traced connection including the trunk circuit HIGTC to the calling subscriber substation in the manual exchange ID.

New assuming that the called subscriber substation T in the automatic exchange 20' is busy when the connector 510 operates in the manner previously explained to select the subscriber line 536' extending thereto, the connector 5H1 operates in order to cause busy tone current to be returned over the previously traced connection including the trunk circuit llilfiTC to the hybrid coil 3ill in the Coupling circuit 300. This audio frequency signalthen passes through the radio link whereby an audio frequency signal, the busy tone signal, is returned over the trunk circuit IGQTC and the cord circuit lfll to the operator headset I I3 at the switchboard I00, in the manner previously explained. At this time the operator at the switch board I00 advises the subscriber at the calling subscriber substation in the manual exchange Iii that the desired connection cannot be had immediately due to the busy condition of the called subscriber substation T in the automatic exchange 20, and then proceeds to withdraw the plugs H83 and PIGZ of the cord circuit [ill from the associated jacks. When the plug P102 is withdrawn from the jack J [5.1 the apparatus involved in the connection extending toward the called subscriber substation T in the automatic exchange 26 is released, in a manner more fully described hereinafter; and when the plug Pl03 is withdrawn from the associated jack the connection to the calling subscriber substation in the manual exchange I!) is taken down.

At the conclusion of the connection, when the subscriber at the calling subscriber substation in the manual exchange l0 replaces the receiver of the telephone instrument thereat upon its associated switchhook, the previously mentioned circuit for maintaining operated the supervisory relay R548 in the cord circuit I01 is interrupted,

whereupon the latter relay restores in order to r complete at the contacts 141, the previously mentioned circuit for illuminating the supervisory lamp L112 in the cord circuit 101. When the supervisory lamp L1 12 is thus illuminated it renders to the operator switchboard 1011 disconnect supervision, whereupon the operator -at the switchboard 100 withdraws the plug P103 of the cord circuit 101 from the associated jack, taking down the connection to the calling subscriber substation in the manual exchange 10,

First assuming that the subscriber at the called subscriber substation T in the automatic exchange 20 replaces the receiver of the telephone instrument thereat upon its associated switchhook before the operator at the switchboard 100 withdraws the plug P102 of the cord circuit 101 from the associated jack J151 terminating the trunk circuit 100TC, the connector 510 operates to reverse the polarity over the trunk '11, the incoming selector 560 and the incoming trunk 520 to the right-hand winding of the answer relay R455 in the trunk circuit 400TC, causing the latter relay to restore. Upon restoring, the answer relay R465 recompletes, at the contacts 466, the previously traced termination circuit, including the resistor 125 and the condenser 418, across the line conductors C402 and C401 extending to the hybrid coil 301 in the coupling circuit 300; and interrupts, at the contacts 461, the previously traced circuit for maintaining operated the supervisory relay R405; thereby to cause the latter relay to restore shortly thereafter, it being of the slow-to-release type. Upon restoring the supervisory relay R485 interrupts, at the contacts 431,

the previously traced path for applying battery potential by way of the resistor 429 to the signalout conductor C403 extending to the coupling circuit 300 in order to interrupt the previously traced circuit for maintaining operated the signal-out relay R315 in the coupling circuit 330. Upon restoring the signal-out relay R315 interrupts, at the contacts 311, the previously mentioned circuit for maintaining operated the hold relay R320; thereby to cause the latter relay to restore shortly thereafter, it being of the slowto-release type. Upon restoring, the hold relay R320 interrupts, at the contacts 321, one of the multiple points in the start circuit of the F. M. radio transmitter 310; however without eifect at this time since the start relay R350 occupies its operated position retaining completed, at the contacts 351, the start circuit of the F. M. radio transmitter 310. Further the signal-out relay R315 upon restoring interrupts, at the contacts 316, the plate circuit of the transmitting amplifier 332; thereby to remove the intermediate 4.5 kc. signal from the modulating circuit of the F. M. radio transmitter 310; whereby the F. M. radio transmitter 310 transmits the unmodulated 33.18 mc. radio frequency carrier.

The F. M. radio receiver 240 receives the unmodulated 33.18 mc. radio frequency carrier; whereby the squelch circuit therein operates tointerrupt the circuit for maintaining operated the start relay R250 in order to cause the latter relay to restore shortly thereafter since it is of V the slow-to-release type and to decrease the sensitivity of the output stage in the F. M. radio receiver 240. Upon restoring the start relay R250 interrupts, at the contacts 251, one of the circuit of the F. radio transmitter 210. Since the 4.5 rnc. component is absent from the output stage of the F. M. radio receiver 240 at this time, the D. C. amplifier 2441s controlled in order to decrease the bias therein; whereby the winding of the signal-in relay R245 is energized sufficiently to cause the latter relay to operate. Upon operating the signal-in relay R245 in the coupling circuit 230 interrupts, at the contacts 246, the previously traced path for applying ground potential to the signal-in conductor C194 extending between the coupling circuit 200 and the trunk circuit TC. When the ground potential is thusremoved from the signal-in conductor C194, the previously traced path for energizing the lower Winding of the signal-in relay R180 in the trunk circuit 100TC is interrupted causing the latter relay to restore. Upon restoring the signal-in relay R180 interrupts at the contacts 182, the previously mentioned multiple circuit for illuminating the busy lamp L153; and recompletes, at the contacts 181, the previously traced circuit for energizing the upper winding of the sleeve relay R160 in multiple with the lower Winding thereof and, consequently, in series with'the winding of the supervisory relay R in the cord circuit 101. When this low resistance circuit is thus completed the supervisory relay R130 operates to recomplete, at the contacts 131, the previously mentioned circuit for illuminating the supervisory lamp L111, thereby to render to the operator, at the switchboard 100 disconnect supervision.

Upon receiving disconnect supervision the operator at the switchboard 100 withdraws the plug P102 of the cord circuit 101 from the associated jack J151, thereby interrupting the series circuit for maintaining operated the supervisory relay R130 in the cord circuit Hit and the sleeve relay R in the trunk circuit 100TC. At this time the supervisory relay R130 restores, interrupting, at the contacts 131, the previously mentioned circuit for illuminating the supervisory lamp L111 whereby the cord circuit 101 is restored to its normal condition. Also, when the plug P102 is withdrawn from the jack J 151, the contacts 151' thereof are operated into open circuit relation, interrupting the previously mentioned circuit for maintaining operated the jack relay R170, in order to cause the latter relay to restore shortly thereafter, it being of the slowto-release type. Upon restoring, the sleeve relay R160 interrupts, at the contacts 161, the previously traced path for applying battery potential by way of the resistor 161 to the signal-out conductor C193 extending to the coupling circuit 200; and recompletes, at the contacts 164, the previously mentioned termination circuit, including the resistor 169, across the line conductors C191 and C192 extending to the hybrid coil 201 in the coupling circuit 200. Also, the jack relay R110 interrupts, at the contacts 113, the previously mentioned original circuit for illuminating the busy lamp L153, thereby to indicate that the trunk circuit 100TC is idle and available -for use at this time.

When resistance battery potential in the trunk circuit 100TC is removed from the signal-out conductor C193, the previously traced circuit for maintaining operated the signal-out relay R215 in the coupling circuit 230 is interrupted. Upon restoring the signal-out relay R215 interrupts, at the contacts 216, the plate circuit of the transmitting amplifier 232 and interrupts, at the contacts 217, the circuit for maintaining operated.

attain-4.:

23 the hold relay R220 in order to-causerthelatter relayto restore shortly'thereafter since it is of the slow-to-release type. hold relay R220 interrupts, at the contacts 221, the start circuit of the F. M. radio transmitter 210; whereby operation thereof is arrested at this time.

When operation of the F. M. radiotransmitter' 210 is arrested, the squelch circuit in the F. Mi radio receiver 340 operates to decrease the sensitivity of the output stage therein and tointer- Upon restoring the rupt the circuit fol-maintaining operated the start relay R350; whereby. the latter relay re-' stores shortly thereafter since it is of the slowto-release type. Upon restoringth-e start relay R356 interrupts, at the contacts 351, the start circuit of the F. M. radio transmitter 310; whereby operation thereof is arrested. Alsothe absence of the 4.3 kc. component in the output stage of the F. M. radio receiver 340 controlstheD. C. amplifier 31M, whereby the bias therein:

is decreased; whereby sufficient current: is supplied to the winding of thesignal-in relay R345 to effect operation thereof. Upon operating the signal-in relay RS -55in the coupling circuit 330 interrupts, at the contacts 346,- the previously traced'path for applying ground potential to the signal-in conductor C40 1 extending between the coupling circuit 3111) and the trunk circuit 400TC. When ground potential is thus removed from the signal-in conductor C404, the previously traced circuit for maintaining operated the signal-in relay R480 in the trunk circuit 40DTC is interrupted causing-the latter relay to restore.

Upon restoringthe signal-in'rel'ay R481 interrupts, at the contacts 481, thepreviously tracedloop circuit xtending by. wayof the incoming:

trunk-520 to the incoming selector 556; and thence by way of the trunk 511" to the connector 510; thereby'to bring about the release of the connector 519-. When the connector 51B-is thus released ground potential therein is removed fromthe control conductor O53A-of thesu-bscriber line 539- extending to the line circuit 535, whereby battery potential in the line circuit 535-is applied to the control conductor 053e, thereby marking.

the subscriber line 5351 extending tothe subscriber substation T as idle tothe connectors in the.

group, including the connector 516, having access,

thereto. Also, when the connector 510 is released ground potential is removed from the controlconductor of the trunk 511, bringing about thereleas of the incoming selector 55D and theconsequent removal of ground potential from the control conductor C52 of the incoming trunk 52 B. Whenground potential isremoved from the control conductor C523-oi the-incoming trunk'. 5213 the previously traced multiple circuits for energizingthe left-hand winding of the-answer" relay R 165 and the upper Winding of the switch relay'R tlfi are interrupted. A1SO,.llp0I1'.1'StOI"-' ing, the signal-in relay R386: recompletes, at the ly thereafter, it being ofthe slow-to-release type;

Further, the hold relay Batu-interrupts; at the contact 22% the previously traced circuit 01:

energizing the lower winding of the switch relay R410, thereby to cause the latter relay to restore. Uponrestoring, the switch relay R 116 interrupts, at the contacts 412, 413 and 415, further points in the previously traced connection between the line conductors C402 and C491 extending to the hybrid coil 3111' in the coupling circuit 309 and the line conductors C521 and C522 of the incoming trunk 5213 extending to the incoming selector 560; and recornpletes, at the contacts 111 and 414, the previously traced normal connections, including the contacts 491 and 499, between the line conductors C 392 and C461 and the line conductors C51 1 and C512 oi the outgoing trunk 5111' accessible to the outgoing selector 550. Further, the switch relay R410 interrupts, at the contacts 418, the previously traced path for applying ground potential to the control conductor C513 of the outgoing trunk 5111; and recompletes, at

th contacts 411, the previously traced normalpath for applying battery potential by way of the impedance 328 to the control conductor C513, thereby to mark the outgoing trunk 510'as idle to the outgoing selectors 550, etc. having access thereto. At this time the trunk circuit 4BOTC is completely released and available for further use.

Now assuming that the operator at the switchboard withdraws the plug P1112 of the cord circuit 101 from the associated jack Jl5| terminating the trunk circuit IODTC before the subscriber at the called subscriber substation T in the automatic exchange 20 replaces the receiver of the telephone instrument thereat upon its associated switchhook, when the plug P102 is thus withdrawn from the jack J 151 the sleeve relay R168 and the jack relay R in the trunk circuit 100'10 and the supervisory relay R in the cord circuit 151 all restore, as previously noted. Upon restoring, the sleeve relay R169 interrupts, at the contacts 161, the previously traced path for applying battery potential by way of the resistor 151 to the signal-out conductor C193 in order to bring about the restoration of the signal-out relay R215' and the consequent restoration of the hold relay R220 shortly thereafter in the coupling circuit 2311. Upon restoring the signal-out relay R215 eifectively disconnects the signal oscillator 231 from the modulating circuit of the F. M. radio transmitter 21!); whereby unmodulated 35.42 mc. radio frequency carrier is transmitted therefrom.

When the F. M. radio receiver 340 receives the unmodulated 35.42 mc. radio frequency carrier, the squelch circuit therein operates to effect the restoration of the start relay R350 shortly thereafter'since it is of the slow-to-release type, and todecrease the sensitivity of the output stage therein. Since the 4.3 kc. component is absent from the output stage in the F. M. radio receiver 358; the D. C. amplifier 34 i is controlled to decrease the bias therein'in order to supply sulficient'current to the winding of the sigma-Lin relay R345" to bring about operation thereof. Upon operating the signal-in relay R345" in the coupling circuit 331.1 interrupts, at the contacts 346, the previouslytraced path-for applying ground potential to the signal-in conductor C48 1 extending between the coupling circuit 3110 and the trunk circuit MBTC. When ground potential is thus "removed from the signal-in conductor C40 1, the previously traced circuit for maintaining operated the signal-in relay R686 in the trunk' circuit tell-TC is interrupted causing the latter relay to restore.-

Upon restoring the signal-in relay R481 0 mpletes, at the contacts 482, a circuit including the contacts 416 and 486 for energizing the upper winding of the supervisory relay R485 and interrupts, at the contacts 48 l, the previously traced loop circuit extending between the right-hand of the answer relay R455 and the connector 51!] in order to bring about the release of the connector 510 and the incoming selector 558, in the manner previously explained. When the connector 519 is thus released in the event the subscriber at the called subscriber substation T has not replaced the receiver of the telephone instrument upon its associated switchhook at this time, the line circuit 535 individual to the subscriber line 530 operates in order to lock the subscriber line 530 out of service and to mark it as busy to the connectors in the group, including the connector 510, having access thereto.

When the incoming selector 560 is thus released ground potential is removed from the control conductor C523 of the incoming trunk 520, thereby to interrupt the previously traced multiple circuits for energizing the left-hand winding of the answer relay R465 and the upper winding of the switch relay R l'lil. At this time the answer relay R465 restores, interrupting, at the contacts 461, the previously mentioned circuit for energizing the, lower winding of the supervisory relay R 585; however, the latter relay is maintained in its operated position at this time, due to the completed circuit for energizing the upper winding thereof. Further, upon restoring,

the signal-in relay R480 interrupts, at the contacts 483, the previously mentioned circuit for maintaining operated the hold relay R420, thereby to cause the latter relay to restore shortly thereafter, it being of the slow-to-release type. Finally, the signal-in relay R880, upon restoring, completes, at the contacts 482, the previously traced circuit for operating the dial relay R440. Upon restoring, the holdrelay R429 brings about the restoration of the dial relay R480 shortly thereafter, as previously noted, and interrupts, at the contacts 424, the previously traced circuit for energizing the lower winding of the switch relay R4!!! in order to cause the latter relay to restore. Upon restoring, the switchrelay R410 interrupts, at the contacts 416', the previously traced circuit for energizing the upper winding of the supervisory relay R485, thereby to cause the latter relay to restore shortly thereafter, it being of the slow-to-release type. Upon restoring, the supervisory relay R485 interrupts, at the contacts 481, the previously traced path for applying battery potential by way of the.-

resistor 429 to the signal-out conductor C403 extending to the coupling circuit 389 in order to interrupt the previously traced circuit for maintaining operated the signal-out relay R3I5 in the coupling circuit 338. Upon restoring the signal-out relay R3l5 interrupts, at the contacts 311, the previously traced circuit for maintaining operated the hold relay R326 in order to. cause the latter relay to restore shortly thereafter since it is of the slow-to-release type. Also upon restoring the signal-out relay R3l5 interrupts, at the contacts 3l6, the plate circuitof the transmitting amplifier 332. Upon restoring the hold relay R328 interrupts, at the contacts 32L the start circuit of the F. M. radio transmitter 310 since the start relay R358 already occupies its restored position; whereby operation of the F. M. radio transmitter 3 l l] is arrested.

When operation of theF. M. radio transmitter 3H] is arrested, the squelch circuit in the F. M.

radio receiver 240 operates in order tointerrupt the previously mentioned circuit for maintaining operated the start'relay R250 in order to cause the latter relay to restore shortly thereaftersince it is Of the slow-to-release type and also to decrease the sensitivity of the output stage in the F. M. radio receiver 24!]. Upon restoring the start relay R250 interrupts, at the contacts 25I, the start circuit of the F. M. radio transmitter 2H! since thehold relay R226 i restored at this time. The absence of the 4.5 kc. signal in the output stage of the F. M. radio receiver 240 controls the D. C. amplifier 2M; whereby the bias therein is decreased in order to cause the current supplied to the winding of the signal-in relay R285 to be increased sufficiently to bring about operation thereof. Upon operating the signal-in relay R245 interrupts, at the contacts 246, the previously mentioned path for applying ground potential to the signal-in conductor C|94 extending between the coupling circuit 209 and the trunk circuit IOBTC. When ground potential is thus removed from the signal-in conductor Cl 94, the previously traced circuit for energizing the lower winding of the signal-in relay Rl8l] in the trunk circuit lflflTC is interrupted, causing the latter relay to restore. Upon restoring, the signal-in relay Rl8l interrupts, at the contacts I82, the previously mentioned alternative circuit for illuminating the busy lamp Ll 53, in order to ex tinguish the busy lamp Ll53, thereby to indicate that the trunk circuit IOUTC is idle and available for further use at this time.

Reconsidering the established connection between the calling subscriber substation in the manual exchange Ill and the called subscriber substation T in the automatic exchange 20, it is noted that the subscriber at the called subscriber substation T in the automatic exchange 28 may render flashing switchhook supervision to the operator at the switchboard I00 in the manual exchange I0 by repeatedly operating the associated switchhook at the called subscriber substation '1. Each time the switchhook at the called subscriber substation T is operated and restored, the

connector 5'!!! operates to reverse and then t restore the polarity over the trunk 51 l the incoming selector 560 and the incoming trunk 520 to the right-hand winding of the answer relay R465 in the trunk circuit 4DDTC, causing the latter relay to restore and then reoperate. Each time the answer relay R465 restores and then reoperates, it interrupts and then recompletes, at the contacts 461, the previously mentioned circuit for energizing the lower winding of the supervisory relay R485; whereby the latter relay follows shortly thereafter the answer relay R465 provided the switchhook at the called subscriber substation T is operated at a proper speed; the supervisory relay R485 being of the slow-to-release type. Each time the supervisory relay R485 restores and then reoperates, it interrupts and then recompletes at the contacts 481, the previously traced circuit for maintaining operated the signal-out relay R355 in the coupling circuit 338; whereby the latter relay follows. Each time the signal-out relay R3l5 restores and then reoperates, it interrupts and then recompletes, at the contacts 3 l i, the previousl mentioned circuit for maintaining operated the hold relay R320; however without effect since the latter relay being of the slow-to-release type remains operated during impulsing. Also each time the signal-out relay R3l5 restores and then reoperates, it interr pts and then recompletes, at the contacts 2'? 3H5, the plate circuit of the transmitting amplifier .332; whereb the signal oscillator 33! is effectively disconnected and then reconnected to the modulating circuit of the F. M. radio transmitter 3H3; whereby pulses of unmodulated 33.18 mc. radio frequency carrier are received by the F. M. radio receiver 245; whereby corresponding pulses are transmitted to the start relay R255; however without effect since the latter relay, being of the slow-to-release type, remains operated during impulsing. Also the pulses are transmitted from the output stage of the F. M. radio receiver 24% through the receiving high pass filter 2 into the receiving amplifier 24-2 in order to cause the signal-in relay R245 to follow in an obvious manner. Each time the signal-in relay R245 operates and then restores, it interrupts and the recompletes, at the contacts 246, the previously traced circuit for energizing the lower winding of the signal-in relay RlBi} in the trunk circuit IUOTC; whereb the latter relay follows. Each time the signal--in relay R185 restores and then reoperates, it completes and then interrupts, at the contacts lill, the previously traced circuit for energizing the upper winding of the sleeve relay R160 in multiple with the lower winding thereof and, consequently, in series with the winding f the supervisory rela Rim in the cord circuit IEH; whereby the supervisor relay RISE}, being of the marginal type, follows. Each time the supervisory relay R130 operates and then restores, it completes and then interrupts, at the contacts !3!, the previously mentioned circuit for illuminating the supervisory lamp Ll 3;

whereby the latter lamp is flashed rendering to the operator at the switchboard I00 recall supervision from the called subscriber substation T in the automatic exchange 2!].

Considering now the extension of a connection between the automatic exchange 20 and the manual exchange i6, and assuming that a call is initiated at the calling subscriber substation T in the automatic exchange 20, the line circuit 535 individual to the subscriber line 536 extending to the calling subscriber substation T operates in order to place battery potential upon the control conductor C533 of the subscriber line 530 and ground potential upon the start conductor C536 extending to the distributor 54!] associated with the group of finder-selector links, including the link illustrated, and comprising the finder 542 and the selector 54! The distributor 540 assigns an idle one of the finder-selector links mentioned, such, for example, as that illustrated, and initiates operation of the finder 542 thereof, whereby the finder 542 operates to seize the subscriber line 539 extending to the calling subscriber substation T bringing about further operation of the line circuit .535, whereupon ground potential is placed upon the control conductor C534 of the subscriber line 538 in order to mark the subscriber line 53!! as busy to the connectors in the group, including the connector 535, having access thereto. Also, the line circuit 535 releases the distributor 548 and the selector 5 5i is conditioned to be responsive to the first digit dialed at the calling subscriber substation T. Furthermore, it is noted that the calling subscriber substation T is included in a ten line group rendered a particular class of service, whereby the set of switch springs S544 inthe finder 542 is actuated into engagement incident to operation of the finder 542 to seize the subscriber line 339 extending to the calling subscriber substation T. When the set of switch springs S544 is thus actuated the class tone conductor C546, to which a class tone indicative of the class of the calling subscriber substation T is subsequentl impressed, is connected by Way of the condenser 545 to the set of switch springs $5 18 in the selector 54!; which set of switch springs S543 is operated further to connect the tone conductor C546 to the control conductor of a seized trunk, only in the event of the selector 54! is operated in accordance with a predetermined first digit indicative of a trunk call. Thus, it will be understood that when the selector 54! is operated to seize the trunk 51! extending to the connector 5'18 when a local call is extended by wa of the finder-selector link illustrated that the set of switch springs S543 is not operated to connect the class tone conductor C546 to the control conductor of the trunk 51'! extending to the connector 518, since such operation would be of no utility in conjunction with the extension of a local call from the calling subscriber substation T.

However, continuing with the extension of the present call from the calling subscriber substation T when the subscriber thereat dials the first digit into the selector 54!, the selector 54! operates in order to select first the correspondin group of trunks and then an idle trunk therein. For example, the selector 54! may operate to select the particular trunk 55! extending to the outgoing selector 558 in response to the first digit dialed therein, whereby the loop circuit from the calling subscriber substation T is extended into the outgoing selector 550. Also, since this is a trunk call the set of switch springs $543 in the selector 54! is actuated into engagement, thereby to extend the class tone conductor C545 over the control conductor of the trunk 55! to the control conductor of the outgoing selector 550. The outgoing selector 555 then responds to the second digit dialed at the calling subscriber substation T, selecting first the corresponding group of out going trunks and then an idle outgoing trunk therein. For example, the outgoing selector 553 may select the outgoing trunk 5!!) extending to the trunk circuit 400TC, whereby there is completed a loop circuit extending from the calling subscriber substation T to the line relay R49fl in the trunk circuit 4G5TC; and the control conductor of the outgoing selector 550 is connected to the control conductor C5|3 of the outgoing trunk 5H], for a purpose more fully explained hereinafter. In passing, it is noted that the outgoing trunk 5!!) is marked as idle to the outgoing selector 550 by the application of battery potential thereon over the previously traced path, including the contacts M3 and 41'! and the impedance 428.

The loop circuit mentioned extends from ground by way of the lower winding of the line relay R49!) and the contacts 499 to the line conductor C5I2 of the outgoing trunk 5H1, and from battery by way of the upper winding of the line relay R490 and the contacts 48'! to the line conductor C5! i of the outgoing trunk 5 l U; and thence by way of the outgoing selector 550, the trunk 55!, the selector 54!, the finder 542 and the subscriber line 530 to the calling subscriber substation T. When this loop circuit is thus completed the line relay R49!) operates to complete, at the contacts 492, a path for applying battery potential by way of the resistor 29 to the signal-out conductor C403 extending into the coupling circuit 355 Also, the line relay R490 interrupts, at the contacts 49L the previously traced termination circuit, including the resistor 425 and the condenser 4l8, across the line conductors C402 

